1. Field of the Invention
The present invention relates to a three-dimensional optical recording medium made of a material whose refractive index varies depending on the light intensity of an optical pulse or a train of optical waves, and an optical information recording apparatus which causes optical pulses or a train of optical waves to meet at a predetermined three-dimensional position in the recording medium in accordance with information to be recorded to form refractive-index changing portions, thereby permitting the information to be recorded or written once.
2. Description of the Related Art
As one type of the conventional optical recording techniques of recording or writing information once on recording media typified by a compact disk and a laser disk, there is a system of recording pits on the surface of a disk-shaped optical recording medium. This optical recording system utilizes a converged laser beam and the heat of the light-absorbed energy to form pits. There is also known a rewritable photo-electromagnetic disk, which utilizes the heat of a laser beam as well as the magnetic rotation to record information as magnetic inversion domains on the surface of the recording film.
Those optical recording systems involve two-dimensional recording media and intermittently scan the surface of such a recording medium with a single converged light beam to perform optical recording information.
Japanese Unexamined Patent Publication No. 2-210627 discloses an optical recording/reproducing apparatus, which uses a three-dimensional recording medium having optical waveguides laminated three-dimensionally and reproduces information from refractive index-discontinuous portions on the recording medium utilizing the condensing property of an objective lens. This recording method forms the refractive index-discontinuous portions as phase-changing portions simply according to the focusing depth of the objective lens, and is generally difficult to record information with an improved spatial resolution below the focusing depth. The refractive index-discontinuous portions may be previously formed as upheavals in the optical waveguide, in which case information cannot be additionally recorded.
The above-described prior art has a physical limitation to improve the surface density of the recording film of a two-dimensional recording medium, and the conventional optical recording methods cannot achieve three-dimensional recording. The last-mentioned method of utilizing the focusing depth of the objective lens cannot improve the spatial resolution.